1. Field of the Invention
The present invention relates to a drive circuit for an ultrasonic motor, and more particularly to a drive circuit for an ultrasonic motor which supplies the ultrasonic motor with drive signals of a predetermined frequency, and controls the frequency of the drive signals to drive the ultrasonic motor.
2. Description of the Prior Art
Conventionally, ultrasonic motors have been known in which ultrasonic vibration is utilized to generate a driving force. A traveling-wave type ultrasonic motor, one type of ultrasonic motor is driven by supplying the piezoelectric body with two-phase signals from a drive circuit, which signals are of respective predetermined frequencies and have a phase difference of 90.degree. therebetween. Due to mechanical vibration of the piezoelectric body generated by the drive signals, an ultrasonic vibration (traveling wave) whose loops and nodes travel in a circumferential direction along the elastic body is excited in the elastic body attached to the piezoelectric body. Due to the traveling wave, a rotor which is press-contacted to the elastic body and a drive shaft are rotated.
The amplitude of the mechanical vibration generated in the piezoelectric body reaches the maximum when the frequencies of the drive signals coincide with the resonant frequency. However, within an audible sound generating band (see FIG. 16) including the resonant frequency, abnormal vibrations of an audible frequency are generated in the elastic body, and the vibrations lower the rotational speed of the rotor and the efficiency of the ultrasonic motor. Accordingly, the ultrasonic motor is driven such that drive signals whose frequencies are sufficiently greater than the audible sound generating band are initially supplied to the ultrasonic motor. The frequencies of the drive signals are then gradually lowered so as to enter a drive frequency band slightly greater than the audible sound generating band and are then maintained within this drive frequency band. However, the audible sound generating band and the drive frequency band of the ultrasonic motor change depending on the ambient temperature of the ultrasonic motor and the magnitude of the load acting on the ultrasonic motor. Accordingly, the frequencies of the drive signals must be varied in accordance with variations in the ambient temperature, load, etc.
As an example of frequency control of drive signals, a technique is disclosed in Japanese Patent Application Laid-open No. 62-203575 in which an AC detection signal output from a piezoelectric element which detects mechanical vibrations of the elastic body is subjected to half-wave rectification by a diode, followed by integration with a resistor and a capacitor. The frequency is controlled such that the level of the integrated signal becomes a predetermined value lower than a predetermined level which is obtained at the above-mentioned resonant frequency. Another technique is disclosed in Japanese Patent Application Laid-open No. 3-159583 in which irregularity of the waveform of a detection signal output from a piezoelectric element is monitored, and the frequencies of drive signals are lowered when irregularity of the waveform does not occur. On the contrary, when irregularity of the waveform occurs, it is judged that the frequencies of the drive signals have entered the audible sound generating band, and the frequencies are raised.
In the frequency control arrangement disclosed in Japanese Patent Application Laid-open No. 62-203575, if the level of the integrated detection signal is lower than the predetermined level, the frequency is lowered even when the frequencies of the drive signals enter the audible sound generating band and irregularity of the drive signals occurs. Accordingly, this frequency control arrangement has the drawback that the frequencies of the drive signals enter the audible sound generating band so that a remarkable audible sound is sometimes generated from the ultrasonic motor. Further, among various ultrasonic motors, there are some ultrasonic motors in which irregularity of the waveform of the detection signal does not occur even when the frequencies of the drive signals enter the audible sound generating band. When the frequency control method disclosed in Japanese Patent Application Laid-open No. 3-159583 is used for driving the above-mentioned ultrasonic motor, the frequencies of the drive signals are lowered and enter the audible sound generating band, and are further lowered, passing the resonant frequency, because irregularity of the waveform of the detection signal does not occur even when the frequencies of the drive signals enter the audible sound generating band. Accordingly, this frequency control method is not suitable for ultrasonic motors in which irregularity of the waveform of the detection signal does not occur in the audible sound generating band.
Meanwhile, the inventors of the present invention have proposed a drive circuit for an ultrasonic motor which has solved the above-described problems, as described in the Japanese Patent Application Laid-open No. 6-178556. In this drive circuit, the level of a detection signal is compared with a predetermined level which is previously set. If the level of the detection signal exceeds the predetermined level, the frequencies of the drive signals are raised only in the period when the level of the detection signal is greater than the predetermined level. On the contrary, if the level of the detection signal falls below the predetermined level, the frequencies of the drive signals are lowered only in the period when the level of the detection signal is lower than the predetermined level. The predetermined levels, and the amounts of variation in the frequencies in each period may be set such that the amount of variation for raising the frequencies in the period where the level of the detection signal is greater than the predetermined level becomes equal to the amount of variation for lowering the frequencies in the period where the level of the detection signal is lower than the predetermined level, for example, in case where the frequencies of the drive signals are set to the optimum drive frequency.
When the frequencies of the drive signals coincide with the optimum drive frequency, as shown in FIG. 17A, an amount of variation by which the level of the frequency control signal Vf is lowered (thereby the frequencies of the drive signals are increased) during the period where the level of a signal V.sub.B obtained by rectifying a detection signal V.sub.A is greater than a predetermined level Vs becomes equal to an amount of variation by which the level of the frequency control signal Vf is raised (thereby the frequencies of the drive signals are decreased) during the period where the level of the signal V.sub.B is lower than the predetermined level Vs. Accordingly, the frequencies of the drive signals are controlled by the frequency control signal V.sub.f to maintain the present values as a whole.
When the frequencies of the drive signals enter the. audible sound generating band, irregularity of the waveform of the detection signal V.sub.A occurs so that the signal V.sub.B mixedly has large amplitudes greatly exceeding the predetermined level V.sub.S and small amplitudes not reaching the predetermined level V.sub.S. As a whole, the periods exceeding the predetermined level become longer due to the large amplitudes, compared to when the driving is performed at the optimum drive frequency. Accordingly, the level of the frequency control signal V.sub.f gradually decreases as a whole, as shown in FIG. 17B, so that the frequencies of the drive signals are varied to become higher as a whole.
In the case of driving an ultrasonic motor in which irregularity of the waveform of the detection signal does not occur in the audible sound generating band, the amplitude of the detection signal V.sub.A increases when the frequencies of the drive signals enter the audible sound generating band, even though no irregularity occur in the waveform of the detection signal V.sub.A (not shown in the drawing). Accordingly, the periods in which the level of the detection signal exceeds the predetermined level V.sub.S become longer, so that the frequencies f of the drive signals are varied to become higher, like in the above-described case. This operation allows driving of the ultrasonic motor in which irregularity of the waveform of the detection signal does not occur in the audible sound generating band.
However, depending on the amplitudes of the drive signals, the frequencies of the drive signals, the magnitude of a load acting on the ultrasonic motor, and the like, there are cases where the periods in which the signal V.sub.B obtained by rectifying the detection signal V.sub.A is greater than the predetermined level V.sub.S become very short, as shown in FIG. 17C, even though the ultrasonic motor generates an audible sound and irregularity occurs in the detection signal. Also, when the rotational velocity of the rotor decreases due to a large load acting on the ultrasonic motor, a considerable difference is produced between the traveling speed of the traveling wave in the stator and the rotational speed of the rotor, so that friction occurs therebetween and abnormal vibrations of audible frequencies are generated at the stator, leading to the generation of an audible sound. In such a state, there are cases where the level of the detection signal is low even though irregularity occurs in the detection signal, and the state continues in which the level of the detection signal is lower than the predetermined level.
As described above, it is desired to increase the frequencies of the drive signals to prevent the ultrasonic motor from generating a remarkable audible sound when irregularity of the detection signal occurs. To the contrary, in the technique disclosed in the above-described Japanese Patent Application Laid-open No. 6-178556, although the frequencies of the drive signals are raised when the level of the detection signal exceeds the predetermined level, the level of the frequency control signal Vf is raised (see FIG. 17C), i.e., the frequencies of the driving signals are further lowered when the amplitude of the detection signal is small so that the periods in which the level of the detection signal is greater than the predetermined level become short, or the state continues in which the level of the detection signal does not exceed the predetermined level. Accordingly, the drive circuit has a problem that it allows the ultrasonic motor to generate a remarkable audible sound.
To overcome the above-described problem, the load acting on the ultrasonic motor and the rotational speed of the rotor may be detected with various sensors to compensate the level of the detection signal or the predetermined level based on the detected conditions. However, in this case, the problem arises that the ultrasonic motor becomes larger and the costs of the ultrasonic motor increases because various sensors must be attached to the ultrasonic motor.